A mechanical seal is a shaft sealing device that contains process fluids within a pump or other type of rotating equipment. There are generally three types of mechanical seals: component seals, made of several pieces; cartridge seals, made of one piece; and split seals. Cartridge seals generally are preferred over component seals because cartridge seals may be installed without significant training and may be tested before shipping to ensure reliability.
Pumps and mechanical seals are utilized by many industries and serve a variety of functions by moving process fluids throughout a plant. For example, pulp and paper manufacturing, chemical processing, petroleum, chemical and oil refining, utilities, and food processing, are among the more significant industries that utilize significant numbers of pumps and associated mechanical seals. Within a large processing plant there may be thousands of different pumps and associated seals, moving a variety of process fluids throughout the plant. The loss of any individual pump within the plant may cause a degradation in the plant output, profitability and efficiency. It also is common for a plant to be reconfigured either to process different products or to provide a work around to avoid a damaged pump. This reconfiguration may result in incompatible combinations of equipment and process fluids and an increased likelihood of failure.
Proper selection, installation, maintenance, operation and failure analysis of rotating equipment, and in particular pumps and mechanical seals, within a processing plant are factors in the reliability, productivity, efficiency and profitability of a processing plant, but are difficult. For example, the selection process of a seal involves the consideration of several factors, such as the operating conditions of the pump, the process fluid to be moved, the type of pump on which the seal is to be installed, and the environmental conditions under which the pump and seal operates. Other factors include the cost and quality of the seal and its ease of installation.
The selection process typically involves a seal or pump manufacturer's trained sales engineers with factory support to ensure that a proper seal is selected. Several standards have been promulgated to establish guidelines for seal selection. These standards include the Society of Tribologists and Lubricating Engineer (STLE) SP-30 1990 and its updated version in April 1994, the CMA/STLE “Mechanical Seal Application Guide” (1994), and the American Petroleum Institute (API) Mechanical Seal Standard 1994. The sales engineer typically has training in mechanical or chemical engineering and is provided by the manufacturer with at least some of the technical data corresponding to the seal or pump products. The sales engineer's effectiveness also may relate to experience in a particular industry. For example, a sales engineer that is experienced in the petroleum industry may not be as effective as proposing solutions for a food processing plant.
Often the selection process is a manual process, prone to errors in communication and understanding between supplier and customer. In addition to communications problems, the different levels of experience among the sales engineers may lead to confusion when different sales engineers working for the same manufacturer make different recommendations based on their experience and understanding of the equipment.
Even if the selection process is accurate for given conditions, improper installation, operation or maintenance of the pump and seals may degrade the operation. A lack of trained personnel often is a factor in improper installation, operation and maintenance of a mechanical seal or pump. In particular, it is possible that a sales engineer without proper training may select an improper seal.
Performance of equipment also should be monitored. To ensure that equipment is operating with acceptable performance, a disciplined, problem solving approach to pump and seal operation and maintenance is used. This disciplined problem solving approach can be divided into eight areas.
The first area is defining an acceptable seal performance metric. By choosing a performance metric that is appropriate for an installation seal, performance can be measured and determined. A performance metric may be, for example, a federal, state, or local government regulation, e.g. limiting emissions from the seal, or the metric may be a measure of frequency or cost of failure, such as a mean time between failures.
The second area is troubleshooting the equipment in the field. Visual observations of the equipment and seal, the seal support system, the piping system, etc., can provide important information and data for later analysis. It also may be possible to provide corrective actions for solving the problem or failure without shutting the equipment down. Examples of such corrective actions include tightening gland bolts and adjusting a quench.
The third area is reviewing the current process and equipment data, along with the repair history for the equipment. This information can provide data on changing conditions that have negatively impacted seal reliability. Because the configuration of the processing plant changes often, it is difficult to maintain data about the configuration of the plant up to date. Modifications to equipment and changes to process fluids commonly occur. Processing plant reliability managers commonly do not have a convenient and timely method of determining the current state of equipment in a plant. In addition, because of the lack of information regarding the current state of equipment within the plant, the plant reliability manager often has inadequate information for cost and failure analysis. Life cycle costs (LCC) and mean time between failure (MTBF) are commonly used metrics to determine the efficiency and productivity of plant equipment. LCC involves tracking the costs associated with a particular pump and/or seal over a given period of time. MTBF involves tracking the time between failures of the particular piece of machinery. Without accurate up to date information on the current state of a piece of equipment, however, these measures cannot be computed accurately.
The fourth area is proper selection of pumps and seals. As pointed out above, seal selection generally is a technically difficult and manual process.
The fifth area is investigating the operational history of the pump and mechanical seal and related equipment. Such an investigation may reveal operating conditions that are incompatible with a seal or pump or other equipment. For example, operating conditions such as pressure, environmental or process fluid temperatures, etc. may deviate significantly from normal operating conditions. By analyzing these deviations from normal operating conditions, the cause of a failure may be determined to have been due to the operating conditions and not due to a mechanical failure. In addition to any data from instrumentation, the personnel responsible for operating the equipment may provide valuable data about any deviations that have occurred and why these deviations occurred.
The sixth area is performing seal analysis after a failure. Disassembly and inspection of a seal helps to understand the failure mode of the seal. There may be mechanical, thermal, or chemical damage to the seal. Information about the failure mode helps to understand the underlying root cause of the failure.
The seventh area is performing a root cause analysis to assign the ultimate underlying cause of the failure based on gathered failure data. The data that has been gathered is analyzed and, based on individual experience and scientific analysis, the root cause of the failure is determined.
The eighth area is implementing a corrective action plan and providing drawings, installation, operation procedures and training to personnel. Monitoring the work performed and updating the equipment databases also may be included in an action plan.
Failure analysis of a rotating equipment therefore is a complex and difficult activity. Often, the processing plant is dependent upon the seal or pump manufacturer to aid in this analysis. The involvement of a manufacturer in the analysis of the cause of a failure of equipment may lead to biased results.
There are other problems with current methods of failure analysis. Even without bias, the analysis is still dependent upon knowledge and experience of the analyst, and thus involves training and retaining personnel. Failure analysis performed in a plant also may fail to consider the pump and seal as part of a system, because a failure typically is examined as an isolated event independent of other considerations. Because of the level of human involvement in the failure analysis, the analysis tends to be experiential rather than scientific. Thus, the analysis is only as good as the experience and insight of the people involved. Without a disciplined approach to gathering data and a scientific basis for analysis, only the symptoms of the failure are addressed and not the underlying root cause of the problem.